The present invention relates to a sealing system for a gas sensor and a method for manufacturing the sealing system.
A sealing system of this type is described in German Published Patent Application No. 198 52 674. This sealing system uses a sealing element made of a mixture of steatite and a glass having a low melting point. The low-melting-point glass includes lead, zinc, bismuth or alkaline-earth metals in the form of oxides, borates, phosphates, or silicates. The sealing system is pressed between two solid-sintered molded ceramic parts and it separates a measuring-gas-side segment of a sensor element, secured in a housing of the gas sensor, from a connection-side segment of the sensor element, the connection-side segment protruding into a reference gas chamber, which is acted upon by a reference gas. To manufacture the sealing system, a compressed sealing ring is inserted into the longitudinal bore hole and is pressed between the two molded ceramic parts. Subsequently, the sealing ring is subjected to a thermal treatment at a temperature between 500 and 700xc2x0 Celsius. As a result of the thermal treatment, the glass powder in the steatite matrix is melted and partially diffuses into the pores of the steatite matrix.
At temperatures in the range of 500xc2x0 Celsius and above, the insulation effect of the sealing system falls off considerably, so that the electrical resistance between the sensor element and housing, given the above-mentioned composition, is reduced to a value of less than 5 Mxcexa9 at a temperature of 500xc2x0 Celsius. Therefore, between the sensor element and the housing, electrical currents may flow, which impair the functioning of the sensor element. In addition, the sealing system is not suitable for temperatures above 700xc2x0 Celsius, because it includes a low-melting-point glass.
The gas sensor according to the present invention may provide the advantage that it has a high electrical resistance between the sensor element and the housing, and it includes a sealing system including a sealing element that is gas-tight even at temperatures above 700xc2x0 Celsius and that is impermeable to fluids, e.g., to fuels. For this purpose, the sealing element includes a mixture of ceramic material and glass, the glass having a hemisphere temperature of over 750xc2x0 Celsius. The hemisphere temperature of a glass is determined by slowly heating a cylindrical figure having a diameter of 3 mm and a height of 3 mm. The hemisphere temperature is the temperature at which the body is deformed by the heating such that the height of the body corresponds exactly to one half of the diameter of the figure, i.e., 1.5 mm.
The method according to the present invention may provide the advantage that the manufacturing of the seal may be integrated in the mass production of gas sensors in a cost-effective manner.
The composition of the glass-forming materials may be selected so that the electrical resistance between the sensor element and the housing, at a temperature of 500xc2x0 Celsius, is greater than 20 Mxcexa9. For this purpose, glass-forming materials may be used which have a high resistance in a composite along with the ceramic material. In contrast, glass-forming materials having a lower resistance in a composite are only used in small quantities. These requirements may be met by a glass that includes a high proportion of barium, strontium, boron, zinc, and/or silicon, e.g., in the form of oxides. In contrast, the glass includes small proportions of iron, copper, lithium, sodium, potassium, magnesium, and/or calcium, also, e.g., in the form of oxides. The proportions of these components altogether are under 8 percent by weight and/or, with regard to individual components, under 5 percent by weight, in each case with regard to the glass. The proportions of these components altogether may be under 5 percent by weight and/or, with respect to the individual components, under 3 percent by weight.
The ceramic material may include steatite, boron nitride, forsterite, aluminum oxide, magnesium spinel, zirconium oxide, or zirconium oxide stabilized using calcium oxide, magnesium oxide, or yttrium oxide, or a mixture of the latter.
A temperature-resistant as well as gas-tight and gasoline-tight seal may be achieved if the sealing element includes a proportion of ceramic material of 45 to 90 percent by volume, e.g., 60 to 80 percent by volume, and a proportion of glass of 10 to 55 percent by volume, e.g., 20 to 40 percent by volume. To avoid mechanical stresses, the composition of the sealing element may be selected so that the thermal expansion coefficient of the glass is between 7xc2x710xe2x88x926 Kxe2x88x921 and 10xc2x710xe2x88x926 Kxe2x88x921, and the thermal expansion coefficient of the ceramic material is between 7xc2x710xe2x88x926 Kxe2x88x921 and 12xc2x710xe2x88x926 Kxe2x88x921.
In an example embodiment, the sealing system is configured in a so-called sandwich arrangement and includes a first, second, and third sealing element, the second sealing element being arranged between the first and the third sealing element. At least one of the sealing elements includes the mixture of the ceramic material and the glass. As further materials for the sealing elements, steatite, boron nitride, or a mixture of steatite and boron nitride may be provided. A sealing system may include a sealing element including steatite and/or boron nitride arranged between two glass-ceramic sealing elements, or a glass-ceramic sealing element arranged between two sealing elements including steatite and/or boron nitride, or a sealing element including steatite and/or boron nitride arranged between a glass-ceramic sealing element and a steatite sealing element. In the sealing systems described, the sealing action is strengthened even more by the combination of different materials.
If the sealing element is prefabricated before it is inserted into the housing of the gas sensor, by pressing the mixture of the ceramic powder and a glass-forming powder in a pressing method forming a sealing ring, and by simultaneously and/or subsequently subjecting it to a temperature of 300 to 600xc2x0 Celsius, then the sealing ring will be strengthened to the point that it has the necessary stability when it is installed in the housing. Because the glass, in this context, is only heated to a temperature significantly below the hemisphere temperature, a plastic deformation of the prefabricated sealing ring is possible under the influence of a pressure force, after installation in the housing. In this context, it is possible, using deformation, to adjust the prefabricated sealing ring to a longitudinal bore hole of the housing and to the sensor element. Subsequently, the preassembled assembly is subjected to a thermal treatment at a temperature of 750 to 1000xc2x0 Celsius, e.g., 800 to 900xc2x0 Celsius, as a result of which the sealing element is formed. In this thermal treatment, the glass-forming powder in the mixture is melted and diffuses at least partially into the pores of the ceramic powder. The thermal treatment may also be performed before and/or during the application of the pressing force.
As a result of the thermal treatment of the mixture of the ceramic powder and the glass-forming powder, it is possible that in addition to the purely crystalline ceramic phases and the purely amorphous glass phases, glass ceramic phases are also formed, which arise due to the crystallization, e.g., of the glass-forming powder. The glass included in the sealing element should be understood as a material that exists in the amorphous glass phase and/or in the crystalline glass ceramic phase that arises under certain manufacturing conditions.
Example embodiments of the present invention are illustrated in the drawings and are described in greater detail below.